With increasing development, liquid crystal displays (LCDs) are widely used due to their advantages of small size, light weight, low driving voltage, low power consumption and good portability. The liquid crystal molecules of the liquid crystal displays are aligned in response to electrical filed changes, thereby controlling the brightness or darkness of the liquid crystal display.
According the production technologies, liquid crystal displays are typically classified into two major types, i.e. a passive matrix liquid crystal display (PM-LCD) and an active matrix liquid crystal display (AM-LCD). In 1970s, the PM-LCDs have been applied in watches and portable calculators. However, since such LCDs have disadvantages of insufficient brightness, narrow viewing angle and low response speed, their applications are restricted. Since the active matrix liquid crystal display (AM-LCD) can drive unitary pixel without influencing adjacent pixels and have good color quality and quicker response speed, they are widely used as liquid crystal displays of digital still cameras, liquid crystal projectors, cellular phones, notebooks, etc. So far, the active matrix liquid crystal displays are relatively popular.
The active matrix liquid crystal displays are generally classified as two major types: diode-structured LCDs and transistor-structured LCDs. Thin film transistor liquid crystal display (TFT-LCD) is one of the transistor-structured LCDs. As is known, TFTs (thin film transistors) are widely used as switching elements for controlling respective pixels of a TFT liquid crystal display (TFT-LCD) so as to achieve good image quality such as high contrasts, wide viewing angles, rapid response, low cost, etc.
The thin film transistors used in the active matrix liquid crystal display is usually a three-terminal TFT with a top-gate typed structure. As shown in FIG. 1, the three-terminal TFT with a top-gate structure comprises a P-channel TFT 1 and an N-channel TFT 2, wherein the N-channel TFT 2 has a lightly doped drain (LDD) region 24. The source region 12 and the drain region 13 of the P-channel TFT 1 are implemented by implanting boron thereinto. Whereas, the source region 22 and the drain region 23 of the N-channel TFT 2 are implemented by implanting phosphorous thereinto. The LDD regions 24 of the N-channel TFT 2 are useful to minimize current leakage. For example, the gate electrodes 11 and 21 are made of molybdenum/tungsten alloy, the gate oxide layer 14 and the insulator layer 15 are made of silicon dioxide, and the conductive lines are made of pure aluminum. Afterward, the resulting structure is covered with a passivation layer 16, which is made of nitride silicon.
As a result of miniaturization, the channel between the source and drain regions in each TFT will become narrower and narrower. Some undesirable effects such as “hot electron effect”, “breakdown voltage” and “impact ionization” are rendered in the vicinity of the drain region 23. Although the LDD regions 24 of the N-channel TFT 2 can reduce the undesirable effects, some additional problems occur. For example, a photo-misalignment problem occurs because at least two masks and at least two ion-implanting steps are required for forming the LDD regions 24. Such photo-misalignment problem impairs electrical properties of the TFT and thus image quality of the liquid crystal display.